The present invention relates to an angle transducer suitable for use with a system for measuring and transmitting a controlled torque from a drive shaft. More specifically, the transducer can be used in a web spooling mechanism which reduces tension transients in a web as the web is wound onto a spool coupled to the chuck.
In order to maximize spooling efficiency, it is important to wind web material on a spool at a very high angular velocity in a very short cycle time. For example, it would be desirable to wind approximately 1.5 to 2.0 meters of photographic film onto a spool at peak angular velocities of about 7,000 rpm in a cycle time of 500 to 700 milliseconds. The film to be wound onto the spool is fed through a pair of metering rolls to the spool, and during the winding operation the portion of the film between the spool and the metering rolls is under tension.
The tension in the film being wound onto the spool may change rapidly for several reasons. More specifically, a film spool typically has a core of a generally cylindrical configuration. However, the spool core may not be precisely cylindrical, especially when the spool is molded from plastic materials. Also, one end of the film may be attached to the core by a strip of tape so that after one convolution of film is wound onto the core the film encounters a sharp change in diameter due to wrapping of the film over the end of the film attached to the core. In addition, the film spool is mounted between a rotatable spool holder, known as a tail stock, which engages one end of the spool, and a chuck which is coupled to the other end of the spool to drive the spool. This mounting of the spool is not precise and, as a result, the spool may be rotated about an axis that is offset slightly from the axis of the core of the spool. Each of these factors result in periodic angular acceleration of the spool and winding mechanism inertia during winding that are generally characterized as "radial runout". Such acceleration induces high frequency periodic tension transients in the film which are linearly proportional to the sum of the spool inertia and the inertia of the mechanism rigidly coupled to the spool. The inertia of the mechanism coupled to the spool is typically several orders of magnitude more than that of the film spool. The factors that contribute to radial runout are clearly undesirable, especially at the very high angular velocities and short cycle times mentioned previously.
If film is wound onto a spool while the film is under an excessive amount of tension so called "pressure marks" can be formed on the film. Pressure marks are formed at the point where the film wraps over the leading edge of the film on the spool while the film is being subjected to excessively high tension. This defect can occur not only in the first convolution of film but in successive convolutions as well. Pressure marks can be avoided by controlling tension in the film.
In some winding and spooling apparatus it is possible to contact the web directly to sense the tension in the web and then control the wind up apparatus to maintain the desired tension in the web. However, in prior film spooling apparatus there is little or no space available for such control apparatus. Also, photographic film is easily scratched by contact-type sensors, and film is not acceptable for use when scratched in the image areas of the film. Accordingly, it is not practical to use such film tension sensing and control apparatus in spooling mechanisms for photographic film.
It is known in the art to provide a torque meter, dynamometer, or the like for measuring the amount of torque being applied from a prime mover to a load. Some such prior systems employ incremental encoders on opposite ends of a torsionally stressed element but process the output in a way which precludes torque angle measurement at zero angular velocity. Such devices are not practical for use with web winding apparatus to solve the problem of isolation of apparatus inertia from a web, and avoiding the problems of runout as discussed above. Thus, it is desirable to provide a transducer that senses and produces a signal indicative of the forces exerted by drive and driven members so that an angle can be measured and used for controlling the drive.